AST-1591: Add option 'usemodeldata' to ApplyBeam
The ApplyBeam step now supports applying the beam to model data through the usemodeldata
option.
An integration test using DDECal to compare the visibilities generated by Predict with usebeammodel
,
and by WGridderPredict followed by an ApplyBeam step yield equivalent results, i.e. all gains found be DDECal are 1.0